Rab Proteins and the Organization of Organelle Membrane Domains - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 23

and indirectly interconnected and thereby a cooperative interaction mesh is created.
This sustains membrane domain formation and further downstream PI/Rab GTPase
effector recruitment.
2.3.3 Rab Effector Proteins Include Membrane Tethers
Molecular tethers guide the specific recognition and binding (tethering) of two
opposing membranes and are thereby a prerequisite for membrane fusion. A key
function of Rab proteins is to recruit such tethers on their respective membrane
domain. Almost all tethers fall into two categories: multi-subunit complexes or
rod-shaped extended coiled coil proteins. Examples of multi-subunit tethers are the
transport protein particle (TRAPP), conserved oligomeric Golgi (COG), depen-
dence on SLY1-20 (Dsl1) and Golgi-associated retrograde protein (GARP) com-
plexes at the Golgi complex, Exocyst at the plasma membrane, and the “class C
core vacuole/endosome tethering” (CORVET) and “homotypic fusion and protein
sorting” (HOPS) complex at early and late endosomes respectively (Bonifacino and
Hierro 2011 ; Br¨cker et al. 2010 ). Examples of extended coiled coil tethers are
EEA1 at endosomes and the Golgin family at the Golgi (Munro 2011 ). Membrane
recognition domains are located on both sides of the tethers to provide membrane
specificity by interaction with, amongst others, PIs and Rab GTPases (Br¨cker
et al. 2010 ). For example, Rab5 recruits a variety of tethers, including EEA1 and the
CORVET complex (Christoforidis et al. 1999a ; Simonsen et al. 1998 ; Peplowska
et al. 2007 ).
EEA1 is an essential component of the endosomal docking and fusion machinery
(Mills et al. 1998 ; Christoforidis et al. 1999a ) and forms parallel coiled coil
homodimers. On the N-terminus, EEA1 contains a Zn 2+ -finger domain through
which it binds to Rab5 and to a lesser extent, the related Rab22 (Kauppi et al. 2002 ;
Simonsen et al. 1998 ; Lawe et al. 2002 ). The EEA1 C-terminus also has a Rab5
binding site and a FYVE domain for binding both Rab5 and PI(3)P (Fig. 2.3c )
(Merithew et al. 2003 ; Simonsen et al. 1998 ). This topology allows for a more
stable recruitment to the PI(3)P-enriched endosomal membrane via the C-terminus
and, therefore, makes it suitable for tethering of incoming Rab5-positive endocytic
vesicles onto endosomes. It should be noted that mutations within the C-terminal
Rab5 binding site did not inhibit EEA1 membrane binding but induced enlarged
endosomes suggesting that the interaction with Rab5 is required for proper endo-
some function (Lawe et al. 2002 ).
The multi-subunit endosomal tethering complex CORVET has only been stud-
ied in yeast so far. The complex consists of four core class C subunits Vps11p,
Vps16p, Vps18p, and Vps33p, which are shared with the late endosomal/lysosomal
tethering complex HOPS, and two CORVET-specific subunits Vps3p and Vps8p
(Fig. 2.3d ) (Peplowska et al. 2007 ). Both Vps3p and Vps8p can interact with
Ypt51p/Vps21p, the yeast homologue of Rab5 (Horazdovsky et al. 1996 ; Plemel
et al. 2011 ; Markgraf et al. 2009 ). By extending recent structural insight into HOPS